re: spots - Pronged spots can be dated back to at least the middle ages, but the problem is documenting them in use in later times until the 1860's. What period are you looking at? I would research horse tack/harness of whatever period as that is the most likely are of use.
re: tacks
the following info is from "The Museum of the Fur Trade Quarterly" Vol 18, Nos 1 & 2, Spring-Summer 1982
.....in the 1770's. Lawrence Ermatinger,one of the Montreal entrepreneurs in the trade to Michilimackinack and the Northwest, sent these orders to Thomas Bridge, in London:
October 1773 - 20M (M = thousand) flemish tacks, 500 in a paper.
October 1774 - 40M (M = thousand) flemish tacks to be packed up in papers of 100
1837 - 20,000 brass tacks for annuities to the Yankton and Santee Sioux'
1839 - "Inventory of Stock at Fort Clark, the Property of Sioux Outfit 1838, transferred to Upper Missouri Outfit 1839." Included "2M Brass tacks."
1843 - An outfit sent from Fort Platte to David Adams on the South Fork of the Platte included "l M Brass tacks.
1864 - Invoice to the Upper Missouri Outfit via Steamer Yellowstone includes 50M Brass tacks.7
1868 - The Peace Commission purchased "l 2M one doz. Papers Brass Tacks" from George H. Kimball & Co. at Fort Phil Kearney."
1878 - Bureau of Indian Affairs purchased "One Hundred and sixty six (166) papers" of "Brass head tacks."
(This is extremely significant because it is the first reference to tacks with brass heads.)
There are also records in the Hudson's Bay Company Archives of shipments of brass tacks to York Factory and Fort Edmonton in the 1860's. George Belden included brass tacks in his list of traders' goods "generally sought for by the savages" in the 1860's. Margaret Carrington noted "brass nails" among the goods being sold in the Ward & Bullockstore at Fort Laramie in 1866.
Examples of Indian items decorated with brass tacks can be found in early paintings and photographs and in many private and museum collections. Tacked pieces in the Museum of the Fur Trade collections include two Sioux knife sheaths, a J. Henry flintlock trade rifle, a captured Leman rifle and captured W. Chance Northwest gun, Tryon Northwest gun from Minnesota, percussion trade gun taken during the Riel Rebellion, Cheyenne and Shoshoni pipe tomahawk stems, Brule Sioux flat pipe stem, Northern Plains Indian elk horn whip handle, Osage "gun-stock" club and a fine carved and painted Osage cradle board.
Notes on Usage: In general terms the tacks were used sparingly pre-1850 (when compared to the later periods when they often literally covered belts and other gear). Therefore the cost isn't as critical if one chooses to use the cast/square shank tacks.
2) Steel vs brass shanks: The best data I have is that steel shank tacks didn't appear until the 1870's - see the 1878 note above and a steel shank tack was found at Adobe Walls, TX with a date of 1874.
Currently the ONLY supplier of solid cast brass (actually bronze), square shank tacks is the Trunk Shoppe
[url] www.thetrunkshoppe.com[/url]. They are not cheap - starting price is $0.45 each - but they are well worth it when one wants to be historically correct to the nth degree. A word of caution: the cast tacks are SOFT - so one must pre-drill (I use an awl and punch a hole). The rough square shanks hold very well.
On the other hand the steel shank are much less expensive and are generally accepted at most events without raising eyebrows.
Solid iron tacks show up on trade lists of the 1820-30's as well. Exactly what they looked like is the questions although based on existing tacks of that period they would be square shank cut tacks similar to currently made iron upholstery tacks.